1. Field of the Invention
The invention relates to a receiver and reception method utilizing multipath-propagated signal components of a signal to be received in order to maximize signal energy to be received.
2. Description of the Related Art
In radio systems, such as cellular radio systems, the conditions under which radio waves propagate typically vary continually, which causes variation as a function of time and place, i.e. fading, in a radio signal. Changes occurring in the impulse response of a channel may be caused either by physical changes in the medium (e.g. variation of the refractive index of the medium as a function of temperature, pressure and partial pressure of water vapour) or changes in the geometry of the connection (movement of the transmitter or receiver or obstacle on the connection).
Fast fading of a signal, which is one form of signal fading, is caused by multipath propagation characteristic of the cellular radio environment, wherein the signal propagates via several different routes between a transmitter and a receiver. Such a channel is called Rayleigh fading channel (comprises only multipath-propagated signal components) or Rice fading channel (a received signal also comprises a stable part, i.e. a directly propagated part or a strongly mirror-reflected part).
At the receiver, the multipath-propagated signal components have different phases because of the different propagation paths. A RAKE receiver utilizes these signal components having different phases. Combining the signal components received by different fingers enables the energy of the received signal to be maximized. A RAKE receiver typically comprises several fingers whose delays are set e.g. to correspond with the delays of different signal components measured from the impulse response of the channel. Methods for setting the delays of RAKE receivers are called code phase acquisition or code acquisition methods and code tracking methods. Typically, code acquisition is carried out first, after which code tracking will follow, wherein the delay values set in code phase acquisition are adjusted. A prior art method for RAKE receiver code phase acquisition is disclosed in patent specification FI982856, which is incorporated herein by reference, or in patent specification WO00/41327.
RAKE receivers are used e.g. in a Universal Mobile Telecommunications System (UMTS), which is a wideband data transmission system wherein frequency resources are assigned using a Code Division Multiple Access (CDMA). In a wideband system, a narrowband user data signal is modulated over a relatively wide band by a spreading code that is more wideband than the data signal. In the UMTS system, several users simultaneously transmit over a single frequency channel, and data signals are separated from each other at the receivers on the basis of a pseudo-random spreading code.
A spreading code usually comprises a long pseudo-random bit sequence. The bit rate of the spreading code is much higher than that of a data signal, and in order to distinguish data bits and data symbols from spreading code bits, the latter are called chips. Each user data symbol is multiplied by spreading code chips. The narrowband data signal then spreads over the frequency band used by the spreading code. The spreading code may be one or more data bits long.
In CDMA systems, a RAKE receiver is synchronized with a spreading code sequence signal-component-specifically. Then, in order to carry out code phase acquisition and code tracking, a spreading code generator of the receiver is typically synchronized according to delay values obtained from maximum points of the impulse response. The problem with synchronization, i.e. setting delays, is that code phase acquisition and code tracking take a lot of calculation capacity because of the large number of samples. Furthermore, when the impulse response does not have distinct maximum points but a wide maximum power range, i.e. a ‘fat finger’ situation, it is difficult to achieve synchronization on the basis of the impulse response determined, according to the prior art, from a signal component received by one finger, and usually some of the energy of the signal to be received is lost.